.calendar {
    display: flex;
    width: 350px;

    border: 1px solid black;
    flex-wrap: wrap;

    div {
        box-sizing: border-box;
        width: 50px;
        height: 50px;
        border: 1px solid black;
        display: flex;
        justify-content: center;
        align-items: center;
    }
}